Vibration isolation mount assemblies, also referred to as engine or powertrain mounts, are used in vehicles to isolate the vibrations produced by the engine and/or powertrain assemblies of a motor vehicle. These assemblies usually are made of two structural members or metal bracket members bonded to a volume of a resilient material, such as rubber, by an adhesive means. These assemblies must be strong enough to withstand the large number of cyclic vibrations associated with engines or powertrains. In addition, the assemblies must be able to withstand the various atmospheric events which cause corrosion of metal components. While satisfying these engineering conditions, the assemblies also must be cost-effective, which means they must be economical to produce from readily available engineering materials associated with automobile manufacture.
Various constructions and methods have been utilized in the past to manufacture engine and powertrain mounts which meet these conditions. One such construction is described in U.S. Pat. No. 4,987,679 to Rau dated Jan. 29, 1991, entitled "Vehicular Powertrain Mount Assembly". The engine mount of this patent is manufactured from a pair of metal brackets which are bonded to a volume of vulcanized rubber sandwiched between the brackets, at least one of which is a stamped cold-rolled steel. The bonding material between the rubber and the metal brackets is a two part epoxy adhesive. While Rau clearly discloses bonding between metal and rubber, Rau does not disclose the use of any precoating on the metal brackets. However, in order to obtain the required corrosion resistance to withstand the hostile environment of an automotive mount while still avoiding premature failure resulting from stress-corrosion mechanisms, it is necessary to utilize some sort of metal preparation, particularly for ferrous alloys such as steel. Typical metal preparations include zinc coating, phosphate coating or other coating procedure, such as E-coat. Typically, E-coats can survive corrosive conditions, but fail in elevated temperature applications and contain undesirable heavy metal additives such as lead. However, it is well known that structures such as taught by Rau fail generally at the interface between the corrosion-resistant metal coating and the metal, resulting in a debonding between the rubber and the metal. The failure typically is not due to the strength of the adhesive, typically an epoxy, but due to the quality of the adhesion at the adhesive-metal coating interface. Coatings such as phosphate offer good adhesion under dry, ambient conditions, but have inferior performance in extreme conditions since phosphate does not provide sufficient protection from corrosive elements. Since Rau teaches a metal to rubber bond interface, a corrosion-resistant coating must be applied to steel, and all known metal coatings utilized to provide the necessary corrosion resistance also pose some environmental concern for the manufacturer.
U.S. Pat. No. 5,030,515 to Ozawa dated Jul. 9, 1991 provides a different solution for bonding rubber to metal. Ozawa teaches bonding metal to vulcanized rubber by spraying coating or dipping a primer consisting of an epoxidized diene to first coat the metal surface. The primer coat is obtained by mixing diene polymer with epoxidizing agent and filler materials in a solvent. After this epoxidized rubber primer coat is allowed to dry (i.e. the solvent is allowed to evaporate), a covercoat adhesive consisting of a halogenated polymer or rubber adhesive composition is applied over the primer coat, which in turn is allowed to dry. The surface containing the covercoat and the rubber surface are brought together and cured by heating at an elevated temperature, 250.degree.-400.degree. F. (121-205.degree. C.). Clearly the reaction involves joining the covercoat to the unvulcanized thermoset polymer by promoting chemical bonding between the covercoat and the thermoset polymer as the polymer and the covercoat are cured together. Curing the covercoat to a vulcanized thermoset polymer would result in weak bonding and poor adhesion between the polymer and the covercoat. Furthermore, it is well known that such high curing temperatures would degrade vulcanized rubbers. All of the examples of Ozawa support bonding the uncured covercoat to an unvulcanized thermoset material as the materials are simultaneously cured.
U.S. Pat. No. 4,079,168 to Schwemmer dated Mar. 14, 1978 discloses coating a substrate with a fusible powdered epoxy resin coating composition and then heat bonding unvulcanized elastomeric compositions to the coated substrate using two stage adhesive primer systems. However, as Schwemmer notes, whole epoxy resins are generally excellent adhesives, they do not readily bond to cured rubber surfaces. When epoxy paint coating is used to join the materials after vulcanization of the rubber elements, the epoxy coating tends to crack after a period of time resulting in corrosion of the exposed underlying surface. Thus, Schwemmer's solution is to vulcanization-bond the elastomeric rubber to the epoxy-coated substrate through the adhesive system. Thus, chemical bonds are formed between the adhesive and the rubber.
However, no reliable system exists for bonding a metal substrate to vulcanized rubber, while providing both a strong bond and corrosion resistance of the interface between the substrate and the vulcanized rubber.